Abstract

To accomplish color constancy the illuminant color needs to be discounted from the light reflected from surfaces. Some strategies for discounting the illuminant color use statistics of luminance and chromaticity distribution in natural scenes. In this study we showed whether color constancy exploits the potential cue that was provided by the luminance balance of differently colored surfaces. In our experiments we used six colors: bright and dim red, green, and blue, as surrounding stimulus colors. In most cases, bright colors were set to be optimal colors. They were arranged among 60 hexagonal elements in close-packed structure. The center element served as the test stimulus. The observer adjusted the chromaticity of the test stimulus to obtain a perceptually achromatic surface. We used simulated black body radiations of 3000 (or 4000), 6500, and 20000 K as test illuminants. The results showed that the luminance balance of surfaces with no chromaticity shift had clear effects on the observer’s achromatic setting, which was consistent with our hypothesis on estimating the scene illuminant based on optimal colors.

To further examine this point, it may be useful to note the luminance balances actually used in experiment 4. Under the 4000 K illuminant, surface chromaticities were shifted redward, so that to achieve an L, M, S average equal to the equal energy white, the luminance balance had to be adjusted by dimming the R colors, yielding an R∶B luminance ratio of 1∶3.7; this is much lower than the ratio (1∶0.69) characteristic of optimal colors of those chromaticities under 4000 K (and slightly lower even than the ratio (1∶2.39) that we obtain for optimal colors of those chromaticities under 20000 K). Similarly, the R∶B luminance ratio under our 20000 K illuminant condition (R∶B=1∶0.149) was far higher than would be appropriate for optimal colors of that chromaticity under 20000 K (R∶B=1∶1.123); it was close to, but still more extreme than, that of such optimal colors observed under 4000 K (R∶B=1∶0.339).

To further examine this point, it may be useful to note the luminance balances actually used in experiment 4. Under the 4000 K illuminant, surface chromaticities were shifted redward, so that to achieve an L, M, S average equal to the equal energy white, the luminance balance had to be adjusted by dimming the R colors, yielding an R∶B luminance ratio of 1∶3.7; this is much lower than the ratio (1∶0.69) characteristic of optimal colors of those chromaticities under 4000 K (and slightly lower even than the ratio (1∶2.39) that we obtain for optimal colors of those chromaticities under 20000 K). Similarly, the R∶B luminance ratio under our 20000 K illuminant condition (R∶B=1∶0.149) was far higher than would be appropriate for optimal colors of that chromaticity under 20000 K (R∶B=1∶1.123); it was close to, but still more extreme than, that of such optimal colors observed under 4000 K (R∶B=1∶0.339).

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